Accurate timekeeping. If the timekeeping got messed up it would be worse, I suspect. A second can make a lot of difference in certain applications.
editor's note: this question was in reference to a post complaining about the nessecity of the leap second.

that’s exactly my point. lets say i wanted to measure the amount of time that has elapsed since yesterday, at this exact hour (it is 3:17 in the morning for me)

if you started counting each second after 3:17 AM on june 30th (yesterday), and counted until exactly 24 hours had passed (60 sec/min * 60 min/hr * 24 hr = 86400 seconds, you would count 86400 seconds) it should logically be 3:17 AM on july 1st. but its not, fuck you! it is 3:17:01.

now counting every second that has passed is a somewhat resource-intensive thing for computer software to do. if we wanted to measure duration a smart way, we would just record the initial time and the final time, and subtract the initial from the final time to get a delta represneting the amount of time that had elapsed between the two

this works fine, but only under the assumption that time doesn’t randomly jump around discontinuously at unpredictable intervals

leap years were bad enough. if you’re measuring time across the unfortunate month of februrary, you can modulo the year by 4 to check if there’s an extra leap day and account for it. do most programmers think of that immediately when writing something as simple as a timer? probably not. do the ones that do over-complicate this relatively simple case to a staggering degree? yes of course

but who tf knows when eggheads are going to get bored and throw an extra second on to the clock? these people think that keeping computers obnoxiously in-sync with the rotation of this planet is more important than the ability for software to sanely measure duration

a second is a million billion years to a computer, in fact, when you sync your system time to some remote server your computer time is within a handful of nanoseconds of the computer times on all computers synced via the same server/pool